Microfluidics employs small capillary channels or microchannels in a solid substrate to perform a wide variety of operations. By employing electrical fields with conductive fluids in the microchannels, very small volumes may be accurately moved, reagents mixed, and the presence of an entity of interest determined. In many applications, the determination is fluorescence. Fluorescence can provide for high sensitivity, allows for multiplexing, by detecting photons at different wavelengths; by appropriate use of combinations of fluorophores one can employ a single light source for excitation. In addition, there are a large number of different commercially available detection devices. In addition, many assays have been developed which depend upon the use of fluorescence, such as DNA sequencing, receptor assays dependent upon expression of green fluorescent protein, immunoassays employing a fluorescent label, etc.
In the case of microfluidics, the use of plastics as the substrate has found application. While plastics have the advantage of ease of fabrication, cost and ready availability, they tend to be fluorescent. In addition, when irradiating a sample with excitation light, light scatter results in a significant background signal, particularly when the excitation pathway and emission pathway are the same. While some systems provide for an obtuse angle between the excitation and emission optical pathways, in the case of microfluidics there are the problems of directing the excitation light to the center of the microchannel, the small amount of sample that will be exposed to the irradiation and the substantially diminished fluorescent signal that one observes at an angle from the microchannel. Also, since the excitation light will encounter the device substrate, there is the further fluorescence of the substrate added to the fluorescent signal.
There is, therefore, an interest in designing new approaches to excitation of fluorophores in microchannels and detecting the fluorescent emission efficiently.